Various types of carboxyl containing polymers of vinylidene monomers with at least one terminal methylene group are disclosed in, e.g., U.S. Pat. No. 4,267,103 (Cohen, 1981). Such polymers may be homopolymers, or copolymers with other vinylidene monomers, of unsaturated polymerizable carboxylic acids such as acrylic acid, maleic acid, itaconic acid and the like. Often copolymers of these acids are crosslinked with small amounts of cross-linking agents. These polymers, especially in the form of their salts, absorb large quantities of water or solvent and, therefore, are useful as, for example, suspending agent, gelling agent or rheological properties controlling agent.
The polymerization of the afore-mentioned monomers is normally carried out in the presence of an organic medium which is a solvent for the monomers but is a non-solvent for the polymers. The organic solvent which may be employed in such polymerization process includes: benzene, xylene, tetralin, hexane, heptane, carbon tetrachloride, methyl chloride, ethyl chloride and the like.
Generally, in this type of solvent polymerization, polymers begin to precipitate from the solution shortly after the initiation; and, as the polymerization progresses, they aggregate in a thick slurry form, retaining large quantities of solvent.
A common problem in such polymerization process resides in the inherent difficulty of separating the solvent from the polymer product. A usual separation method such as centrifugation or filtration may not be effective due to the lack of sufficient density difference between the solid rich phase and the solvent or due to the insufficient development of the particle size to utilize a filter. As a result, a direct drying system such as a rotary vacuum dryer is commonly used to separate and recover the solvent from the polymer.
As is well known, however, a vacuum drying system, when used to dry a slurry mixture containing a large quantity of solvent, e.g., in excess of 50% by weight, requires a long drying time. Such a lengthy drying process is not only less economical but also entails a number of undesirable side effects. The polymers, when exposed to a high temperature for a prolonged duration, become vulnerable to thermal degradation or glassification; and may coagulate in various non-uniform sizes of particles. Furthermore, such a conventional drying system, regardless of the length of the drying time, is often unable to remove the solvent residues which are trapped in the interstices of the polymeric particles. Since most of these organic solvents are known to be carcinogenic, the industry has been searching for a viable method capable of producing a polymeric product containing a commercially acceptable level of solvent residue.